Strong Exciton-photon Coupling in Semiconductor Microcavities

The basic building block of vertical cavity surface emitting lasers (VCSELs) and high efficiency diodes, is a quantum well embedded in a semiconductor microcavity. The high finesse that may be achieved in such a cavity is utilised to get a low threshold current in the VCSELs and a high directionality of the spontaneous emission in the diodes.At low temperatures, a strong coupling between the excitons in the quantum well and the electromagnetic field in the cavity may be achieved. This coupling leads to new quantum mechanical states, so-called polaritons, the properties of which are still not fully understood. We present our recent results on microcavities with a record high ratio of the polariton splitting to the line width. The dependence of the polariton line widths on the amount of exciton-photon mixing is shown, which demonstrate that narrowing of the lower polariton state is taking place. Due to the steep dispersion, the amount of phase space available for polariton scattering is reduced, yielding longer dephasing times and hence narrower lines. The possibility of tailoring the polariton dispersion in order to reduce the line width is very interesting, for instance for all-optical switches based on semiconductor microcavities.